X-ray therapy electronic portal imaging system and method for artifact reduction

ABSTRACT

X-ray therapy EPI artifact reduction and control of imaging is provided. Scanning of images is synchronized with the pulse rate of the x-rays. The scanning period is longer than the pulse rate period, so artifacts are generated within the resulting images. Due to the synchronization, the pulse variation artifacts are aligned across multiple images. The synchronization and resulting alignment of linear artifacts allows for gain correction as a function of lines within the image. Such gain correction reduces or removes non-linearities associated with pulse rate variation.

BACKGROUND

The present invention relates to x-ray imaging and dosimetricmeasurements. In particular, reduction of linear accelerator pulsingartifacts for electronic imaging devices of x-rays are provided.

X-ray treatment uses the therapeutic application of x-ray energy todestroy tumor tissue or for other therapy. X-rays generated by amegavoltage or other high voltage sources generate x-ray pulses orperiodically vary the amplitude of the x-rays output, such as every 5milliseconds.

X-ray imaging detectors output signals responsive to the incidentx-rays. The variation in the x-rays results in image artifacts. Wheremultiple images are combined or averaged, the pulse rate variationartifacts are randomly averaged or combined. The resulting combinedimage also undesirably includes artifacts.

X-ray images generated digitally are used for dosimetric treatmentverification. The pulse rate variation artifacts introduce anon-linearity within the images. The artifacts adversely affectmeasurement and image diagnosis of x-ray therapy.

BRIEF SUMMARY

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. By way ofintroduction, the preferred embodiments described below include methodsand systems for x-ray therapy with reduced pulse rate artifactsvariations or removal of x-ray pulsing effects from EPI. Scanning ofimages is synchronized with the pulse rate of the x-rays. The scanningperiod is longer than the pulse rate period, so intensity artifacts aregenerated within the resulting images. Using the synchronization, thepulse variation artifacts are aligned across multiple images. Thesynchronization and resulting alignment of linear artifacts allows forgain correction as a function of lines within the image. Such gaincorrection reduces or removes non-linearities associated with pulse ratevariation.

In one aspect, a dosimetric therapy system for artifact reduction isprovided. An x-ray source has an output responsive to an x-ray pulserate. An imaging device is responsive to x-rays from the x-ray source.The imaging device has a scan trigger input connected with the output ofthe x-ray source.

In a second aspect, an interface system is provided for synchronizing anelectronic x-ray imaging device with pulses of an x-ray machine. A lowdose circuit responsive to an x-ray source high voltage power-on signaland a radiation off signal is operable to generate a first triggersignal and a second trigger signal. The first trigger signal isresponsive to the x-ray source high voltage power-on signal, and thesecond trigger signal is responsive to the radiation-off signal. A highdose circuit is operable to generate a third trigger signal synchronizedwith an x-ray pulse signal.

In a third aspect, an interface system for synchronizing an electronicx-ray imaging device with pulses of an electronic x-ray machine isprovided. An input connects with a trigger circuit. An output alsoconnects with the trigger circuit. An output signal responsive to aperiodic input signal on the input is provided on the output.

In a fourth aspect, a method for artifact reduction in x-ray therapysystems is provided. A sequence of x-ray pulses are generated. Imagingis performed in response to the x-ray pulses during generation of thex-ray pulses. The imaging is synchronized with the x-ray pulses.

In a fifth aspect, a method for artifact reduction in dosimetric therapysystems is provided. An image with linear pulse artifacts is generated.The image is gained corrected as a function of a line.

Further aspects and advantages of the invention are discussed below inconjunction with the preferred embodiments.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures like reference numerals designate correspondingparts throughout the different views.

FIG. 1 is a block diagram of one embodiment of an x-ray therapy system.

FIG. 2 is a block circuit diagram of an interface system of oneembodiment for synchronizing an electronic x-ray imaging device with anx-ray machine.

FIG. 3 is an alternative embodiment of a block circuit diagram forsynchronizing an electronic x-ray imaging device with an x-ray machine.

FIG. 4 is a timing diagram of one embodiment representing operation ofan x-ray therapy system in a low dose mode.

FIG. 5 is a timing diagram of one embodiment representing operation ofan x-ray therapy system in a high dose or continuous scan mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Image scans of a digital x-ray imaging device are synchronized withpulses of x-rays from an x-ray source. Synchronization aligns theresulting pulsing artifacts within multiple images. Location of theartifacts is known, and the combination of multiple images results indistinct artifact patterns. The artifacts are associated with linearpositions within the images, such as horizontal lines across images. Bycontrolling gain as a function of line, the x-ray pulse variationartifacts are removed or reduced. Electronic readouts provideinformation for stable, accurate dosimetric measurements.

FIG. 1 shows an x-ray therapy system 10 of one embodiment. The x-raytherapy system 10 includes an x-ray source 12, and interface 14, and animaging device 16 with a display 18. Additional components, such as apatient bed, motors, components for Intensity Modulation Radio Therapy(IMRT) or other therapeutic x-ray components, may be included. Inalternative embodiments, the interface 14 comprises part of the x-raysource 12 or the imaging device 16. In one embodiment, the x-ray therapysystem 10 comprises a PRIMUS® system with digital imaging capabilitiesfrom Siemens Medical Systems. X-ray therapy systems from othermanufacturers may be used.

The x-ray source 12 comprises a megavoltage or other voltage linearaccelerator for generating x-rays for medical treatment. Depending onthe intended medical therapy, lower energy, mid-energy or high energylinear accelerators may be used. The x-ray source 12 has a fixedposition or is operable to be moved through multiple positions, such asassociated with IMRT. Additional features may be provided for the x-raysource 12, such as combination with CT scanners, combination with otherimaging devices, multi-leaf collimators, or other systems or devices.

The x-ray source 12 outputs x-rays at a selectable energy level formedical imaging and therapy treatment. The x-ray source 12 also outputsdata or information signals representing operation of the x-ray source.In one embodiment, a high voltage power-on signal, a radiation-on andoff signal, and a radiation or x-ray pulse signal are output. The highvoltage power-on signal indicates that the x-ray source 12 is powered onor switched on for application of x-rays. The radiation-on and offsignal indicates that x-rays are being generated or not generated by thex-ray source 12. The x-ray pulse signal indicates the pulse rate orperiod for variable generation of x-rays. For example, when theradiation is on, a pulse signal is provided every 5 millisecondscorresponding to the x-rays pulsing off at 5 millisecond intervals. Thepulse rate information is provided in synchronization with actualpulsing of the x-ray source 12.

In alternative embodiments, different, additional, or fewer outputinformation signals are provided. For example, the radiation on and offsignal is multiplexed with or includes the pulse rate information. Asyet another example, data indicating a pulse rate or period for use by atimer is provided prior to turning-on the radiation.

The imaging device 16 is synchronized with the x-ray source 12. Theimaging device 16 comprises an Electronic Portal Imaging Device (EPID)or other large area flat panel digital x-ray imaging detector forradiographic application. For example, a Beamview® EPID imaging systemfrom Siemens Medical Systems is used, but imaging devices from othermanufacturers may be provided. In one embodiment, a two-dimensionalscintillator or phosphor screen converts x-rays to light. Atwo-dimensional active matrix of photo-detectors or thin filmtransistors made of amorphous or polycrystalline silicon or othersemiconductor materials converts the light energy into electricalenergy. Readout electronics of the active matrix scan the photodiodes toacquire electrical imaging data. The amount of electric charge generatedby the photodiodes or other x-ray detectors is linearly related to theamount of radiation or the photon count received at the imaging device16. Each scan or readout from the two-dimensional array of the activematrix provides an associated plurality of pixel informationrepresenting a two-dimensional area. The information represents the sumof or total amount of radiation provided at locations on thetwo-dimensional array since a previous readout cleared or reset thestored electric charges.

The image information is stored and processed to provide a per pixelindication of radiation dosage at each received pixel. One or moreprocessors, application specific integrated circuits, logic devices oranalog circuits controls the scanning, storing and processing of theimage information. For example, a control processor causes a pluralityof frames or scans of image information to be combined to form a singleimage, such as averaging or summing a plurality of frames of informationassociated with a single or multiple therapeutic dosages of x-rays. Asanother example, the processor applies offset correction to account fordark current or bias currents of the transistors or active matrix, meanor pixel correction to allow for software correction of defectivepixels, and gain correction to homogenize different pixel sensitivities.In one embodiment, gain correction is applied as a function of linerelative to the two-dimensional imaging array. For example, datarepresenting a line within the two-dimensional region is increased ordecreased relative to other lines of data for removing or reducingartifacts from pulse rate variations or linear accelerator pulsingeffects. In alternative embodiments, dedicated hardware or separateprocessors perform any one or more of the various imaging processingfunctions described above or other imaging processes.

The display 18 of the imaging device 16 is a flat panel or CRT monitor.Projection, photographic or other displays may be used in alternativeembodiments. The display 18 generates an image based on the image dataacquired by the imaging device.

The imaging device 16 also includes an external scan trigger input 28.The imaging device 16 scans or generates image data in response to atrigger signal applied to the external scan trigger input 28.Additionally or alternatively, the imaging device 16 scans the activematrix as a function of internal triggers, such as timing signals.

The imaging device 16 is synchronized with the pulse rate of the x-raysource 12 by connecting the pulse signal with the external scan triggerinput of the imaging device 16. The pulse indications trigger imaging bythe imaging device 16. Alternatively, data is input to the imagingdevice 16 indicating a start of radiation, and expected timing of pulsesor a pulse period data provide synchronization information.

In one embodiment, the interface device 14 converts signals output bythe x-ray source 12 into a format usable by the imaging device 16 at theexternal scan trigger input. Additionally or alternatively, theinterface 14 provides additional control for triggering image scanning.In yet other alternative embodiments, a signal output by the x-raysource 12 is connected directly to the scan trigger input 28 of theimaging device 16.

FIG. 2 shows an interface 14 of one embodiment. The interface 14includes a controller 20, a low dosage circuit 22, a high dosage circuit24, an OR gate 26 and the trigger output 28. In alternative embodiments,additional, different or fewer components may be included. For example,only the high dose circuit 24 is provided.

The controller 20 comprises a transistor, a switch, a processor, logicdevice, analog device, software switch or other device for controllingor selecting the low or high dose circuits 22, 24. The controller 20determines a mode of operation of the interface 14. The controller 20switches between the high dose mode and the low dose mode. For example,the controller 20 selects between generation of trigger signals for lowor high dosage readout or triggering. For low dose trigger generation,trigger signals are generated for patient localization imaging, such asto establish the appropriate positioning of a patient relative to thex-ray source 12. The high dose mode provides trigger signals forsubstantially continuous scanning. The trigger signals are synchronizedwith the pulse rate signals from the x-ray source 12. After each triggersignal is generated, other pulse signals from the x-ray source do nottrigger until the end of the frame (i.e. a selected frame has beenscanned or a particular frame period has elapsed). Trigger signalsduring the frame scan period are ignored by the imaging device 16.Subsequently, another trigger signal is generated in synchronizationwith another pulse signal from the x-ray source 12. The controller 20 isresponsive to user selection or indication of the operation of the x-raysource 12, such as selection of positioning imaging or therapeuticimaging.

In alternative embodiments, additional control, such as timingadjustment or other control is provided by the controller 20. In yetother alternative embodiments, the controller 20 controls one, or threeor more trigger generation circuits.

The OR gate 26 passes trigger signals from either of the low dosecircuit 22 or the high dose circuit 24. In alternative embodiments, thetrigger signals from the low and high dose circuits 22, 24 are providedseparately to the imaging device 16 or are combined by connection ofsignal lines or other logic circuits.

The low dose circuit 22 synchronizes the imaging device 16 with thex-ray source 12. The low dose circuit 22 comprises a pair of optocouplers 30, 32, monostable multi-vibrators 34, 42, astablemulti-vibrator 36, delay generator and monostable multi-vibrator 46, ANDgates 38, 40, and 44 and an OR gate 48. Additional, different or fewercomponents may be provided. The low dose circuit 22 generates triggersignals to refresh the imaging device 16 and to trigger generation of animage scanned after application of x-ray radiation has ended.

The opto-couplers 30 and 32 isolate the low dose circuit 22 from thex-ray source 12 for receiving a high voltage power-on signal and theradiation on/off signal. Three multi-vibrators 34, 36, 42 and two ANDgates 38, 40 generate a trigger signal for refreshing or clearing theimaging device 16 (FIG. 1) in preparation for imaging from x-rays. Thetwo monostable multi-vibrators 34, 42 generate a high or low signal fora particular time period in response to a high or low or changing inputvoltage. Accordingly, the monostable multi-vibrators 34, 42 act as pulsewidth circuits or circuits for generating a timing or pulse signal.Other pulse circuits with or without multi-vibrators may be used.

In response to the high voltage being powered on, the monostablemulti-vibrator 34 switches to a different output, such as switching to ahigh output, for one millisecond or other time period. The astablemulti-vibrator 36 generates a square waveform or other periodicwaveform. In one embodiment, the square waveform has a 350 millisecondperiod, such as the same as or greater than a scan rate of the imagingdevice 16.

The AND gate 38 receives the output of the monostable multi-vibrator 34The multi-vibrators of the interface 14 comprise latches or other logiccircuits and associated resistors, variable resistors, capacitors andinductors form controlling the timing of operation of the latch circuitand the control signal from the controller 20. The control signalenables operation of the low dose circuit. If the interface 14 isoperating in the low dose mode and the monostable multi-vibrator output34 is switched high, the AND gate 38 outputs a high signal to themonostable multi-vibrator 42.

The monostable multi-vibrator 42 switches to or latches a high outputfor 1600 milliseconds or other amount of time for a refresh time. Thenext AND gate 40 receives the signal from the refresh time monostablemulti-vibrator 42 and the astable multi-vibrator 36. The monostablemulti-vibrator 42 enables the AND gate 40 for output during the refreshtime. The output of the astable multi-vibrator 36 generates periodictrigger signals while the AND gate 40 is enabled for refreshing theimaging device 16 a plurality of times during the refresh period. Giventhe 1,600 millisecond refresh period and the astable multi-vibrator 350millisecond cycle, four refresh scan triggers are generated. The refreshperiod is a function of the difference in time between the high voltagepower-on and the radiation-on. In one embodiment, about two seconds areprovided between the high voltage power on signal and the application orgeneration of x-ray. Other relative timings are possible.

The low dose circuit 22 also generates a trigger signal after x-rayradiation is turned-off using the AND gate 44 and the delay generatorand monostable multi-vibrator 46. When the radiation is turned off andthe low dose mode is enabled by the controller 20, the AND gate 44generates a high or activation signal to the delay generator andmonostable multi-vibrator 46. A high to low transition of the radiationon signal indicating radiation-off is used to trigger the delaygenerator and monostable multi-vibrator 46. The delay generator andmonostable multi-vibrator 46 comprises two monostable multi-vibrators,but other devices may be used. In response to an activation signal, adelay is implemented prior to latching out a monostable trigger signalto the OR gate 48. For example, resistors and capacitor values areselected for implementing a 1.5 millisecond delay, but other delays maybe used including no delay. The delay compensates for phosphorpersistence of the scentilator screen.

The OR gate 48 receives trigger signals from either the monostablemulti-vibrator 42 responsive to the high voltage power-on signal or thedelay generator and monostable multi-vibrator 46 responsive to theradiation-on and off signal. The OR gate 48 passes the trigger signalsto the OR gate 26.

FIG. 3 shows the interface 14 with a modification to the low dosecircuit 22 for creating two or more trigger signals after the radiationis turned-off. The delay generator and monostable multi-vibrator 46enables a high output by another monostable multi-vibrator 50 after adelay. This other monostable multi-vibrator 50 latches high for a pulsewidth of 800 milliseconds, but other pulse widths corresponding to thedesired number of scans by the imaging device 16 (FIG. 1) may beprovided. The output of the astable multi-vibrator 36 or anotheroscillating signal from another source is input with the pulse widthenabling signal of the monostable multi-vibrator 50 to the AND gate 52.The AND gate 52 outputs two or more trigger signals as a function of thepulse width of enablement provided by the monostable multi-vibrator 50and the frequency of the oscillating signal from the astablemulti-vibrator 36. For example, two trigger signals are generated wherethe monostable multi-vibrator as a pulse width of 800 milliseconds andthe astable multi-vibrator has a 350 millisecond cycle. Other relativetiming relationships may be used, and other combinations of high or lowenabling outputs and inputs may be used.

Referring to FIG. 2, the high dose circuit 24 receives x-ray pulse rateor pulse signals and generates synchronized trigger signals at theexternal scan trigger input 28. The high dose circuit dose circuit 24includes a TTL converter and opto-coupler 54, monostable multi-vibrator56 and an AND gate 58. Additional, different or fewer components may beused. The high dose circuit 24 generates trigger signals while x-raysare generated by the x-ray source 12. The x-ray pulse rate signals areprovided only when x-ray radiation is generated. Alternatively, theradiation-on signal is provided to the high dose circuit 24 for enablinggeneration of trigger signals substantially continuously duringapplication of therapeutic x-rays. Any signal, such as pulse-I, or dosesignals 1 or 2 of the x-ray source 12 indicating pulse timing may beused.

The TTL converter 54 converts the signals into a TTL level logic high orlow signals. The opto-coupler 54 isolates the high dose circuit 24 fromthe x-ray source 12.

In response to the beginning of a pulse or a change to a high or lowvoltage of the pulse rate signal, the monostable multi-vibratorgenerates a trigger signal. The pulse width of the monostablemulti-vibrator 56 and associated trigger signal is 30-40 microseconds,but greater or lesser pulse widths may be used. Other relative timingsmay be used.

The AND gate 58 is enabled by the controller 20. Where the high dosemode is active and the monostable multi-vibrator 56 generates a triggersignal, the AND gate 58 passes the trigger signal to the OR gate 26. TheOR gate 26 passes the trigger signal to the external scan trigger input28. The trigger signal is synchronized with the pulses of the x-raysource 12. Accordingly, pulse variations of the x-ray source 12 occur ata same time for each scan. A linear artifact at the same line or lineswithin each scan is generated due to the synchronization of the pulseswith the scan.

The multi-vibrators of the interface 14 comprise latches or other logiccircuits and associated resistors, variable resistors, capacitors andinductors form controlling the timing of operation of the latch circuit.Inverters and high or low voltage activation of any of the variouscomponents may be used. In an alternative embodiment, an applicationspecific integrated circuit, processor, analog components or both analogand digital components may be used for implementing one or morecomponents of the interface device 14.

FIG. 4 shows a timing diagram for the low dosage x-ray or positionimaging operation of the x-ray therapy system 10. The delivered x-raydoses are at a lower dose for patient positioning or IMRT treatment. Thesystem 10 is idle at time period 70. In response to user control, thehigh voltage power of the x-ray source 12 is turned on at time 71. Inresponse, the x-ray source 12 generates a high voltage power-on outputsignal. The interface 14 generates 1 or more trigger signals provided tothe imaging device 16 during the refresh time period 72. In response tothe trigger signals, the imaging device 16 scans the active matrix torefresh or reset the imaging device 16. Immediately after or after adelay from the refresh time period 72, the x-ray source 12 generatesx-ray radiation during time period 74. The x-ray radiation is generateda set time after the high voltage power is turned-on or in response to arefresh completion signal from the imaging device 16 or interface 14.

When the x-ray source 12 ceases generation of x-ray radiation, theradiation-on signal is turned off or a radiation-off signal is turnedon. The interface 14 generates a trigger signal after a 1.5 milliseconddelay or other delay from the radiation being turned-off. Regardless ofthe signal used to initiate the delay, the delay delays generation of atrigger signal after the radiation is off to compensate for scintillatorscreen persistence. Since scanning by the imaging device 16 is avoidedduring application of the radiation, the imaging device 16 integratesthe light signals generated by the scintillator screen during the entireexposure time 74. At the end of the delay, the interface 14 generatesone or more scan trigger signals. During the imaging time period 76, theimaging device 16 initiates and completes one or more scans or framereadouts. By avoiding frame readouts during application of x-rayradiation and due to the integration of x-ray energy by the photodiodes,good signal to noise ratio and minimal linear accelerator pulsingartifacts appear on the resulting image. Other relative time periods andmodes of operation for the low dose mode may be used.

For IMRT or other multi-position x-ray therapy, multiple frames ofinformation associated with multiple positions of the x-ray source 12are acquired. Where larger intervals of time are provided between eachtreatment, additional frames of information may be scanned after eachapplication of radiation. The characteristics of the imaging device andassociated active matrix may limit the number of scans performed betweenapplication of x-rays from different positions.

During processing time period 78, the information scanned is processed.The imaging device 16 applies offset corrections, gain is corrected as afunction of pixels to homogenize different pixel sensitivities and amean or pixel correction provides software correction of defectivepixels. The gain correction data for the low dose mode is acquired in afree running mode of the imaging device 16 where the imaging device 16continuously generates frames (e.g. 50-100 frames) of informationaccording to a programmed time not synchronized with the x-ray source 12and with radiation but no patient. The offset correction data isacquired as an average of frames (e.g. 100 frames) where no radiation istransmitted. The offset correction frame is used to account for darkcurrent or bias current of transistors used in the active matrix whereno x-ray radiation or associated light is detected by a particulartransistor or photo-detector. If more than one frame is scanned duringperiod 76, these frames are averaged at time 78. Different, fewer oradditional image processing may be provided. The resulting image is usedfor analysis, such as to verify a position of a patient for applicationof therapeutic x-rays.

FIG. 5 shows a timing chart representing one embodiment of operation ofthe x-ray therapy system 10 in a high dose mode of operation forcontinuous scanning while applying x-rays. The x-ray source 12 generatesa high dosage x-ray radiation for therapy. Substantially continuousimaging while radiation is being applied allows for measurement of theamount of radiation dosage and effects of therapy. The x-ray source 12and associated pulse rate output signal are idle during time period 80.At time period 82, the x-ray source 12 generates radiation fortherapeutic application. A linear accelerator pulse signal is output attime 82. In response to the first pulse signal, a trigger signal isgenerated for refreshing the imaging device 16 during time period 84.The imaging device 16 discards or ignores the first frame which is therefresh frame and displays the subsequent frames. The refresh period 84is the same time as scanning periods 86, but may be different. After therefresh time period 84 is complete, no or some delay represented at 83is provided until the next pulse rate signal or linear accelerate pulsesignal is output by the x-ray device 12 as represented at 82. In oneembodiment, the linear accelerator pulse rate is every 5 milliseconds.The refresh time period 84 is 342.5 milliseconds, but other time periodsmay be used. FIG. 5 only represents linear accelerator pulses or thepulse rate signals 82 used for synchronization.

After the refresh period 84 and any associated delay 83, image scanningis triggered in response to the next pulse rate signal 82. The imagingdevice 16 scans to acquire image information for one two-dimensionalframe of data during the scan period 86. After the scan is complete, noor some time period 83 occurs before the next pulse 82 in application ofthe x-ray radiation.

While the radiation is applied and continues to pulse, a plurality ofscans of image data are synchronized with the linear accelerator pulsesas represented by the pulse trigger at 82, the scan period at 86 and anyassociated delay after scan at 83. Based on the synchronization with thepulses of the x-ray source 12, linear intensity artifacts are generatedin each of the scans. The linear intensity artifacts occur at a sameposition within each scan as a function of the synchronization. Forexample, where a scan period 86 occurs over a 300 millisecond timeperiod and the x-ray source pulses the x-ray radiation at a 5millisecond time period, approximately 60 lines within each scannedimage are associated with pulsing artifacts. The pulsing artifacts occurat a same location in each image. In one embodiment, the linearartifacts occur at a same linear horizontal positions spaced evenly ineach two-dimensional representation frame of data. The scan period 86 istriggered to begin at the beginning of each or a subset of the x-raylinear accelerator pulses 82. The trigger pulses generated by the highdose circuit 24 have a width of approximately 30-40, microseconds, butother timing is possible.

The imaging device 16 removes the pulse artifact from the acquired imageby linking to a gain correction image acquired in a continuous scanmode. The gain correction image is a previously acquired average of oneor more frames (e.g. 100 frames) corresponding to synchronizedcontinuous scan with application of x-ray radiation without a patientbeing present. Both pixel sensitivity differences and linear intensityartifacts are present in the gain correction image. The averaged gaincorrection image is used to determine amplitude adjustment as a functionof pixel and line to homogenize or equalize the pixel values of the gaincorrection image. An intensity change due to linear intensity artifactsis determined. The determined homogenization function or amplitudeadjustments are applied to frames of data acquired in the high dosemode. Data representing lines within the two-dimensional regionassociated with the artifact is reduced to remove the linear intensityartifacts. Alternatively, data associated with lines of thetwo-dimensional region free of the artifact are increased. In yetanother alternative embodiment, data associated with artifacts isdecreased and data associated with no artifact is increased.

In one embodiment, two or more images acquired during the high dose scanmode are combined by summing, averaging or other filtering. Due to thesynchronization, the resulting combined image data is associated withpulsing artifacts in the same linear or one-dimensional locations. Gaincorrection is applied to remove the pulsing linear artifact. Combining alarge number of frames of data during the gain correction process alsoreduces the effect of dosage rate variations on the acquired images. Forexample, the dosage rate of the x-ray source 12 varies independently ofthe pulses for about the first two seconds of application of theradiation. Combining a greater number of images associated with scanningbefore and after the two second dosage variation increases accuracy ofgain correction. For example, a 1% deviation due to dose rate variationfor 50 MU exposure is provided. For dosage exposure greater than 50 MU,the accuracy may be increased further.

The image data is used for accurate dosimetric measurement. Given thereduced of artifacts due to dosage variation and pulsing rate variationsand the linear response of the imaging device to applied x-rays,accurate dosimetric measurements are provided, such as amount of x-rayradiation applied or x-ray application area.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. Forexample, different output signals, interface devices and trigger signalscan be used for synchronizing the imaging device with the x-ray source.

It is therefore intended that the foregoing detailed description beunderstood as an illustration of the presently preferred embodiment ofthe invention, and not as a definition of the invention. It is only thefollowing claims, including all equivalents, that are intended to definethe scope of this invention.

1. An interface system for synchronizing an x-ray imaging device withpulses of an x-ray machine to limit artifacts, the system comprising: aninput operable to be connected with the x-ray machine; a low dosecircuit responsive to an x-ray source high voltage power-on signalreceived at the input and a radiation-off signal received at the input,the low dose circuit operable to generate a first trigger signal inresponse to the x-ray source high voltage power-on signal and togenerate a second trigger signal in response to the radiation-offsignal; a high dose circuit responsive to an x-ray pulse signal receivedat the input, the high dose circuit operable to generate a third triggersignal synchronized to the x-ray pulse signal; and an output connectedwith the low and high dose circuits and operable to be connected withthe x-ray imaging device for operation in response to the first, secondor third trigger signals.
 2. The interface system of claim 1 wherein thehigh dose circuit comprises a pulse width circuit operable to generatethe third trigger in response to the x-ray pulse signal.
 3. Theinterface system of claim 1 wherein the low dose circuit comprises firstand second pulse width circuits, the first trigger signal responsive toa first pulse width of the first pulse width circuit and the secondtrigger signal responsive to a second pulse width of the second pulsewidth circuit.
 4. The interface system of claim 1 further comprising acontroller connected with first and second AND gates, the first AND gateconnected with the low dose circuit and the second AND gate connectedwith the high dose circuit.
 5. The interface system of claim 1 furthercomprising an OR gate connected with outputs of the low and high dosecircuits.
 6. An interface system for synchronizing an x-ray imagingdevice with pulses of a x-ray machine to limit artifacts, the systemcomprising: an input from the x-ray machine separate from an x-raydetector; a trigger circuit connected with the input; and an outputconnected with the trigger circuit, an electronic panel scanning triggersignal to be provided on the output responsive to an input signal on theinput, the output operable to be connected with the x-ray imaging devicefor operation in response to the electronic panel scanning triggersignal.
 7. The interface system of claim 6 wherein the trigger circuitcomprises a monostable multivibrator.
 8. The interface system of claim 6further comprising a controller connected with an AND gate, the AND gateconnected with the trigger circuit and the output.
 9. A method forartifact reduction in an x-ray therapy system, the method comprising:(a) generating a sequence of dosage x-ray pulses; (b) imaging inresponse to the dosage x-ray pulses during (a); and (c) synchronizing(b) with the dosage x-ray pulses as a function of a signal being inputto an imaging device, the input signal being separate from x-rayemissions.
 10. The method of claim 9 wherein (b) comprises scanning aplurality of images in response to a respective plurality of triggersignals and (c) comprises generating the plurality of trigger signals asa function of beginnings of the x-ray pulses.
 11. The method of claim 10wherein (c) comprises generating the plurality of trigger signals as afunction of less than all of the beginnings of the x-ray pulses.
 12. Themethod of claim 9 further comprising: (d) identifying a linear artifact;(e) gain correcting images of (b) as a function of a one-dimensionalline associated with the linear artifact.
 13. The method of claim 9wherein (a), (b) and (c) comprise operating the dosimetric system in ahigh dose mode, the method further comprising: (d) operating thedosimetric system in a low dose mode: (d1) generating an x-ray pulse ofless dosage than the x-ray pulses of (a); (d2) imaging after (d1).
 14. Amethod for controlling imaging in an x-ray therapy system, the methodcomprising: (a) generating low dosage x-ray radiation, the low dosageadapted for verifying patient position; (b) preparing an x-ray sourcefor (a); (c) triggering a scan of an electronic portal imaging deviceprior to (a) in response to (b); and (d) scanning in response to (a)after (c) and without generating an image between (b) and (a).
 15. Themethod of claim 14 further comprising: (e) avoiding scanning of theelectronic portal imaging device during (a).
 16. The method of claim 14further comprising: (e) delaying scanning of the electronic portalimaging device for a time period after x-ray radiation of (a) ceases;and (f) scanning the electronic portal imaging device after the delay of(c).
 17. A method for controlling imaging in an x-ray therapy system,the method comprising: (a) generating low dosage x-ray radiation, thelow dosage adapted for verifying patient position; (b) avoiding scanningof a electronic portal imaging device during (a); and (c) scanning theelectronic portal imaging device after (a).
 18. The method of claim 17further comprising: (d) delaying (c) for a time period after x-rayradiation of (a) ceases.
 19. The method of claim 17 further comprising:(d) scanning the electronic portal imaging device prior to (a).
 20. Amethod for synchronizing between a megavoltage linear accelerator forgenerating x-rays and an electronic portal imaging device, the methodcomprising: (a) providing intensity modulated radiation therapy; (b)generating x-ray pulses with the megavoltage linear accelerator; (c)reading data from the electronic portal imaging device, the dataresponsive to the x-ray pulses; and (d) synchronizing betweenmegavoltage linear accelerator and the electronic portal imaging devicesuch that the x-ray pulses are not generated during (c), thesynchronizing being a function of a signal output from the megavoltagelinear accelerator or input to the electronic portal imaging deviceseparate from the x-ray pulses.
 21. The method of claim 20 wherein (d)comprises synchronizing with an interface device.
 22. The method ofclaim 20 wherein (d) comprises synchronizing as a function of a periodicpulse signal of the megavoltage linear accelerator output separate fromthe x-ray pulses.
 23. The method of claim 20 wherein (d) comprisesstarting (b) in response to a control signal.
 24. The method of claim 20wherein (d) comprises triggering the reading of (c) as a function of apulse signal of the megavoltage linear accelerator.
 25. A system forlimiting artifacts for patient verification in intensity modulationradiation therapy related imaging, the system comprising: a megavoltagelinear accelerator; an electronic portal imaging device; an interfaceoperable to synchronize generation of x-ray pulses with the megavoltagelinear accelerator with reading dosage from the electronic portalimaging device, the dosage responsive to the x-ray pulses, the interfaceoperable to communicate between the megavoltage linear accelerator andthe electronic portal imaging device with control signals separate fromthe x-ray pulses.
 26. The system of claim 25 wherein the interface isoperable to generate a control signal for the megavoltage linearaccelerator.
 27. The system of claim 25 wherein the interface isoperable to generate a control signal for the electronic portal imagingdevice.
 28. The system of claim 25 wherein the interface is operable tosynchronize as a function of a pulse signal electronically outputseparate from the x-ray pulses by the megavoltage linear accelerator.29. The system of claim 25 wherein the interface is operable tosynchronize such that the x-ray pulses are not generated during thereading of the dosage.